In a Positron Emission Tomography (PET) apparatus or the like, a radiation position detector provided with a scintillator array and a light detector array is used. The scintillator array includes a plurality of scintillators which are two-dimensionally arranged, and each of the scintillators absorbs radiation and generates scintillation light. The light detector array includes a plurality of light detectors arranged to correspond to the plurality of scintillators, and each of the light detectors outputs an electrical signal in response to the intensity of the scintillation light. For example, in Thomas Frach, et al., “The Digital Silicon Photomultiplier System Architecture and Performance Evaluation”, Nuclear Science Symposium Conference Record, IEEE, 2010, pp. 1722 to 1727, a method of calculating the centroid position of positions where scintillation light is detected on the basis of an electrical signal output from a light detector is described.